The European Space Agency’s ExoMars Trace Gas Orbiter will continue the search for life on Mars.
Photograph: D. Ducros/AFP/Getty Images

The launch of The European Space Agency’s (ESA) ExoMars Trace Gas Orbiter spacecraft on Monday 14 March is a significant moment in space exploration. It marks a new beginning in the search for life on Mars.

Interest in life on Mars stretches back centuries. The 17th century Dutch astronomer Christiaan Huygens speculated in his book Cosmotheoros about the conditions necessary for life. He saw no reason that Mars or the other planets could not be inhabited.

He called the inhabitants ‘Planetarians’ and gave the opinion that their height would be dictated by the size of the planet, so that they were large enough to move around with ease – as we do on Earth.

In the 18th century William Herschel studied Mars, noting the bright polar caps and assuming that the dark markings on the planet were probably seas. He thought that Earth and Mars were somewhat similar, writing in the Philosophical Transactions of the Royal Society that the Martian inhabitants “probably enjoy a situation similar to our own.”

A century later, astronomers were thinking something different. Telescopes were improving all the time and astronomers were seeing what appeared to be straight dark lines connecting the dark ‘seas’.

American astronomer Percival Lowell popularised the notion that these were canals. Mars was mostly desert, he thought, and these canals were the last ditch efforts of a civilisation fighting to irrigate their dying world.

The idea provided inspiration for HG Wells and his novel The War of the Worlds, in which the Martians abandon their fight at home and invade Earth instead.

It is now thought that the ‘canals’ were optical illusions produced by slightly inferior telescopes. As soon as technology improved, the lines were never seen again.

By the 20th century, analysis of Mars’ atmosphere showed that there was hardly any water vapour present, and this put paid to the idea that there were large bodies of water there. The arrival of the first spacecraft in the 1960s showed that there had been water in the distant past and this eventually led to our current ideas that microbial life could have formed billions of years ago and that perhaps some is still clinging on.

In the late 1970s, Nasa’s two Viking missions touched down on the planet. Each lander contained a biological laboratory capable of performing four different experiments that could reveal life. Although the Labeled Release experiment gave initially positive results, it failed when repeated.

The other three experiments showed only negative results and so officially Nasa declared that no life was detected. Controversially, some scientists continue to claim that the initial success of the Labeled Release experiment should be treated as proof of life.

Regardless of such claims, Nasa have been moving away from looking for life on Mars. Instead, they have concentrated on investigating the evidence that Mars was once more Earth-like with flowing water and a thicker atmosphere.

The latest twist began serendipitously in 2003. Esa’s Mars Express spacecraft made a tentative detection of methane in the planet’s atmosphere. Methane is a short-lived gas, and on Earth it is produced mainly by lifeforms.

In 2014, Nasa’s Curiosity rover also detected whiffs of the gas. This increased speculation that perhaps there were microbial communities still alive on Mars, producing the gas. The recent detection of running water on Mars also upped the stakes.

However, there is also the chance that the methane is produced by geological processes.

Esa’s ExoMars Trace Gas Orbiter is designed to find out which of these options is the most likely. It will look for the gas with unprecedented sensitivity and map its distribution across the planet over a number of years. If anywhere is particularly rich in methane production, that will be a natural target for further investigations.

On which note, Esa is sending the ExoMars rover to Mars’s surface in 2018. It will be equipped with instruments capable of detecting the signs of past and present microbial life.